1. Field of the Invention
The application relates to a method and a related communication device used in a wireless communication system and related communication device, and more particularly, to a method of handling close subscriber group (CSG) list synchronization in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
In the LTE system, a home base station, sometimes called a femto base station or a pico base station, is considered to be a promising solution for many issues in mobile communication networks, such as coverage, capacity, and cost. The home base station delivers a great user experience in homes, office buildings or schools, at very low cost, using customer's broadband access (e.g. digital subscriber line (DSL) or cable television backhaul). For this application, the home base station usually serves small cellular coverage that has a restricted group of users and only allows access and provides service to the restricted group of users, which can be referred as to a close subscriber group (CSG). Such cells providing the small cellular coverage are referred to as femto-cells, Home Node B (HNB) cells, Home e-Node B (HeNB) cells or, most commonly, CSG cells.
The CSG cell is accessible only for a limited set of UEs (i.e. closed subscriber group). In order to prevent a UE from attempting to register on a CSG cell on which they do not have access, the UE maintains an allowed CSG list, i.e. a list of CSG identities (ids) for which access has been granted to the UE. The CSG id is an identifier broadcast by the CSG cell or cells and used by the UE to facilitate access for authorized members of the associated closed subscriber group. It shall be possible to store the allowed CSG list in the USIM (Universal Subscriber Identity Module) of the UE. When available, the allowed CSG list on the USIM shall be used. It shall be possible for both, an operator and a user, to modify the allowed CSG list. Based on home operator preference, the use of the allowed CSG list may be inhibited. The operator preference dictates if the allowed CSG list in a ME (mobile equipment) shall be used or not. On the other hand, the UE shall maintain an operator controlled list of allowed CSG identities (e.g. operator CSG list). It shall be possible to store the operator CSG list in the USIM of the UE. When available, the operator CSG list on the USIM shall be used. It shall be possible for the operator to modify the operator CSG list.
All the CSG cells sharing the same identity are identifiable as a single group for the purposes of mobility management and charging in a PLMN (public land mobile network). A VPLMN (Visit PLMN) may provide CSG membership to the roaming subscriber without exchanging any CSG specific information with a HPLMN (Home PLMN). The HPLMN shall be able to instruct the UE so that it will not attempt to access CSG cells not contained in the allowed or operator CSG lists while in a VPLMN. The HPLMN shall be able to configure this restriction on a per VPLMN basis.
According to subclause 5.3.2 of section 1 in 3GPP specification TS 22.220 Va.3.0, the operator may inhibit the use of Allowed CSG List based on the operator preference. In this case Manual CSG selection to a CSG ID which is not in the operator CSG list is not possible as a UE can't add this in the Operator CSG List. Hence, when a user subscribed CSG IDs list is not in sync between a UE and a network, then the user will loose the service of subscribed CSG cells which CSG IDs are not in the UE's Operators CSG List due to loss of an OTA (over-the-air) message intended to update the UE's Operator CSG list. The user can't add this CSG ID of these CSG cells to the Operator CSG List through manual CSG ID selection procedure and there is no way to let the network know that the list of CSG IDs in UE's Operator CSG List. This will cause loss of subscribed CSG Cell services to the user for unspecified period of time.
For a manual CSG selection, the user shall be able to request the UE to perform a scan for available CSGs. When such request is received the UE shall perform a scan of available CSGs, their CSG Identities and their HNB Names. In the UE display, the available CSGs shall be represented by their associated HNB Names and PLMN Name(s). According to subclause 5.3.1 of section 1 in 3GPP specification TS 22.220 Va.3.0, a HPLMN can instruct a UE not to access CSG cells not contained in the allowed or operator CSG lists while in a VPLMN. The HPLMN shall be able to configure this restriction on a per VPLMN basis. According 3GPP specification, however, the UE will display all available CSG IDs at that location during manual CSG selection procedure. The user can select the CSG ID which is not in its Allowed CSG List or Operator CSG List. The UE will then try to access this CSG cell by initiating Registration Procedure which is not desired by the HPLMN in some VPLMN as per requirement described above. So, displaying CSG IDs not belonging to Allowed CSG List or Operator CSG List to user in manual selection will defeat the requirement.